The invention disclosed relates to roller or wheel units in earth-moving vehicles, of a type which require limiting means to check end float of the rotatable body, and where pre-loading of such end float needs to be more than somewhat precise.
An application in which the invention would be employed to advantage is that of rollers or wheels turning on rolling bearings, which become subject to not inconsiderable thrust loads when the machine is in operation; a typical example would be the bottom rollers of crawler vehicles. In such an application as this it is good engineering to pair the roller and shaft together with a pair of taper roller bearings located between the two; such bearings not only give long service life, but offer notably good resistance to thrust loads.
The invention is similarly applicable however in the case of roller or wheel units turning on ball bearings, a typical example of which is that of the top rollers of crawler vehicles, and is applicable likewise in the case of idler wheels.
This much asserted, it is nonetheless quite possible for ball bearings to be fitted to bottom rollers and idler wheels, and for taper roller types to be fitted to top rollers.
In conventional crawler track rollers having rolling radial bearings (whether taper roller or ball type), the outer ring of the bearing is generally integral with the inner surface of the roller component (which consists of a hollow body having a longitudinal bore of section significantly larger than the section of the shaft), whilst the inner ring fits positively over the shaft.
In order to inhibit movement of the roller body through an axial direction, along the shaft, one has the inclusion of internal limiting means which generally take the form of shoulders located in the bore of the roller body, and prevent the outer ring of the bearing from sliding in toward the center, and of external limiting means, generally taking the form of annular brackets, which prevent the inner ring of the bearing from drifting outwards. In certain instances, the internal limiting means will consist of shoulders located in the shaft, which check the bearing inner ring, whilst the external limiting means consist of annular brackets integral with the roller body, which check sliding movement of the outer ring.
A person skilled in the art will know that correct operation of radial bearings, and especially of taper roller bearings, depends on permissible end float being set with extreme precision, that is: too short a relative axial distance between the cone (the inner ring) and the cup (outer ring) of the bearing will cause their opposed surfaces likewise to be too tightly spaced, and rollers located between the two will be unduly compressed; difficulty in producing rolling motion ensues, as a result of which excessive friction is generated. Where the same relative axial distance is long, poor operation also results, this time occasioning early wear of the taper rollers at certain key points. In short, relative axial distance between cone and cup must be absolutely precise in order to ensure optimum performance of the bearing, and this is a condition which automatically dictates a high degree of precision in selecting the relative axial positions of the internal and external limiting means, since these in effect establish the relative axial distance between the cone and cup of the taper roller bearing.
The prior art affords substantially two different methods of ensuring the aforesaid precision.
In a first method, the distance between the internal limiting means (shoulders incorporated into either the roller body or the shaft) is fixed, whereas external limiting means are provided which can be adjusted for position, by way, for example, of lock-nuts screwed onto the shaft, or of brackets bolted to the roller body the position of which can be altered by introducing shims; in this way one obtains the correct relative position between the cup and cone of each bearing by compensating for the axial machining tolerances of the separate components. Such a method is effective enough insofar as acceptable pre-load of the bearing float is obtained, and correct operation thus ensured; nevertheless, costly and complex parts are needed(locknuts, threads, shims and c.), and besides, each roller requires adjusting singly, involving a lengthy and by no means sample procedure.
The second method makes use of fixed-distance internal limiting means as above, and of similarly fixed-distance external limiting means (brackets set on the shaft which abut with snap rings located in grooves machined in the shaft itself). With this type of method it is clearly essential that axial machining tolerances envisaged for the separate components must be considerably tight, as a result of which it is impossible, in practice, to ensure good bearing pre-load; bearings thus operate in hostile conditions and early wear is occasioned, especially in the case of taper roller types. The upshot of such a situation is that new bearings must be fitted at least once during the life of the roller, signifying drawbacks in terms of the vehicle being out of service, of the work involved, and of expense on parts. The bearings, moreover, operating in such hostile conditions, are unable to withstand radial or thrust loads of a high order, and the use of this type of unit in crawler tracks is thus limited to top roller applications for which loads are lighter than those on bottom rollers. At all events, the components still require precision machining, and production costs are therefore high.
The problems and drawbacks described above are experienced generally in all units featuring a shaft and a rotatable body--that is, either highly accurate machining is required in order to build in the requisite pre-load, or, special means must be utilized in order to compensate for axial machining tolerances.
The object of the invention is one of embodying a roller or wheel unit which features low manufacturing costs, by virtue of the fact, first, that it requires no precision machining, and second, that assembly is rendered swift and simple; such a unit is also highly dependable by reason of the precision obtained in thrust pre-load.
Applied to rollers or wheels with rolling bearings, thrust pre-load according to the invention is highly accurate and obtains the optimum setting; bearings are thus able to provide maximum specified service life by virtue of their operating in ideal conditions. Accurate pre-load also permits of the bearing's withstanding high loads, thrust and radial alike, signifying that the roller can be employed in the most heavy duty of conditions (e.g. bottom roller in a tracked vehicle).
With wear on bearings reduced to a minimum, the life of the bearing will either match or outstrip that of the roller itself, the result being that the roller will not normally require any servicing which calls for repair or replacement of bearings.
Thrust pre-load according to the invention is extremely precise, and certainly more accurate than that obtained normally with those prior art methods aforementioned designed to compensate for machining tolerances.